This invention relates in general to steam turbines. More specifically, the present invention is directed to structural arrangements for steam turbines. The turbine arrangement disclosed eliminates stress corrosion cracking with a novel structural arrangement including a shrinkage-fitting rotor.
Various types of steam turbine rotors are known. These include: rotors manufactured from integrally forged alloy steel or similar raw material by mechanical working of the steel or raw material; rotors manufactured by welding disc-shaped raw material to produce an integral whole, which integral whole is then subjected to mechanical working; and rotors produced by shrinkage-fitting onto a rotor shaft a disc in which blades have been anchored after completion of mechanical working. Of these various known turbine arrangements, the shrinkage-fitting type has become widely accepted because large rotors can be manufactured from forged material of comparatively small dimensions, since the material is divided between the rotor shaft and a plurality of discs.
FIG. 1 (PRIOR ART) shows an example of a typical shrinkage-fitted rotor. A plurality of discs 3, each of which has a multiplicity of blades 2 mounted about its periphery, are fitted onto a rotor shaft 1. For each disc 3, the internal diameter d.sub.1 of the disc is made smaller, at room temperature (based on known shrinkage fitting diameter proportion relationships), than the external diameter d.sub.2 of the rotor shaft at the location of mounting of the disc on rotor shaft 1. When discs 3 are to be coupled to rotor shaft 1, only the discs are heated and expand thermally so that the internal diameter d.sub.1 of a particular disc 3 becomes larger than the external diameter d.sub.2 of rotor shaft 1 at the mounting position. With the internal diameters of discs 3 thus expanded, the rotor shaft 1 is inserted through each of the discs, positioned appropriately, and the discs are then made to contract by cooling them, thereby fixing them securely to rotor shaft 1.
Disc bore keys 4 are provided at the junction of each of discs 3 and rotor shaft 1, so that even if, under exceptional turbine operating conditions, the shrinkage fit should loosen, the discs cannot move relative to the rotor shaft 1.
Problems exist in the use of known shrinkage fit rotors, as described above. In such shrinkage-fitted rotors, there is a risk of development of corrosion cracks in the rotor shaft or in the discs due to stress under the special steam environment which exists during operation of the turbine. These corrosion cracks may shorten the rotor life and adversely affect its reliability.
One mechanism which results in the development of such stress corrosion cracks involves local breakdown of a surface oxide layer of the metal in the oxygen-containing water or steam environment which exists during turbine operation. This causes cracks due to selective dissolving of the affected metal portions under the action of tensile stress on the material. Stress corrosion cracking occurs when there is a coincidence of the following three factors: sensitivity of the material to cracking; stress higher than a critical value; and the material being placed in an environment wherein local formation and breakdown of the oxidation layer occurs.
The sensitivity of the material to stress corrosion cracking has a close relationship with the strength of the material. In general, the higher the tensile strength of the material, the greater is its sensitivity to cracking. However, for shrinkage-fitted rotor discs, low alloy steel of high tensile strength must be used because of the high stress to which the discs are subjected. This results in a high cracking sensitivity. It is believed that, in the future, selection or development of materials that have no cracking sensitivity at all will be practically impossible.
Also, the discs of a shrinkage-fitted rotor are subjected to a shrinkage-fitting stress caused by the initial shrinkage fitting of the discs to the rotor, and to centrifugal stress accompanying the rotation that acts on the discs themselves and on the blades. The value of this stress increases in the radially inwards direction of the discs. In particular, stress concentration occurs, due to the shape, in the regions of the key grooves where the disc bore keys 4 used for position locking the discs to the rotor shaft 1 are mounted. This stress may often exceed the critical value for stress-induced corrosion cracking.
The properties of the steam in a power generating installation in which the turbine is used are fixed by the overall design specifications of the reactor, boiler or the like steam generating equipment and the condensing plant or water-supply, etc., used in the power generating installation. It is therefore difficult to sufficiently control water quality to prevent stress-induced corrosion cracking of discs 3.
Thus in general, due to the combination of the three factors of the material, stress, and environment in the neighborhood of the key grooves of the discs of a shrinkage-fitted rotor, there is a considerable likelihood of occurrence of stress-induced corrosion cracking. If such stress corrosion cracks occur in the discs, and are not detected before rotor use by non-destructive testing etc., they may even lead to destruction of the discs 3.